1. Field of Invention
The present invention relates to power converters, and more particularly, to a power converter with hysteric control.
2. Description of Related Art
Power converters are essential for many modem electronic devices. Among other capabilities, power converters can adjust voltage level downward (buck converter) or adjust voltage level upward (boost converter). Power converters may also convert from alternating current (AC) power to direct current (DC) power, or vice versa. Power converters are typically implemented using one or more switching devices, such as transistors, which are turned on and off to deliver power to the output of the converter. Power converters may also include one or more capacitors or inductors.
In some applications for power converters, the load current may vary significantly (e.g., over several orders of magnitude), in which case it is desirable to have rapid response in the regulation or control of the converters. One form of regulating a power converter is hysteretic control, also know as bang-bang control or ripple regulation. In a typical implementation, a hysteretic regulator will turn on the switching device of a power converter when Vout is below a certain voltage (e.g., 5V), and will turn off the switching device of the converter when Vout is above the voltage.
Hysteretic regulators offer advantages over loops containing a compensated error amplifier, known as “linear control” loops. One advantage is very fast load transient response, which minimizes the required output capacitor value. Another advantage of a hysteretic regulator is that no loop compensation components are required. But hysteretic regulators have certain drawbacks. For example, such regulators are prone to unstable behavior with an output capacitor (Cout) having low equivalent series resistance (ESR). More specifically, when Cout has a small value of ESR, the output voltage Vout of the converter will not rise with the switching device turning on, thus resulting in chaotic behavior. This unstable behavior results from the feedback not being in phase with the inductor current.
To address such drawback, a power converter according to one previously developed design uses a transconductance (GM) amplifier to sense the voltage across the inductor. Such converter then creates a ramp using a capacitor and sums it with a DC error voltage from an error amplifier. A disadvantage of this previously developed design is that it requires the use of a separate error amplifier, thus resulting in a more complicated control to implement hysteretic regulation.